No other values should be changed in this band-pass filter except the L and C.

I was trying to get a natural/resonance frequency the same as the input which is 1 kHz, and the following are the values I have chosen: L = 25.33 μH and C = 1 mF, however, I got an output plot that is not a band-pass filter. Why is that so?

enter image description here

  • \$\begingroup\$ try a sweep with smaller steps: .ac lin 10000 950 1050 \$\endgroup\$
    – glen_geek
    Commented Dec 30, 2022 at 17:35

2 Answers 2


Your simulation is only running between 100 KHz and 10 MHz.

Try editing the command to:

.ac dec 1000 10 100K

Besides the problem with the sweep range already mentioned, your values look odd.

You have a rather large capacitance and a rather small inductance.

When you run the sweep you're going to find that there is a lot of loss at the center frequency, around 14 dB. If you reduce the capacitance and increase the inductance you will reduce the loss. The trade off is that the bandwidth will be increased.

You can try dividing the capacitance by powers of 10 and multiplying the inductance by the same value. For example now you have 1000 uF and 25 uH, try 100uF and 250 uH, 10 uF and 2.5 mH, or 1 uF and 25 mH and see how the L to C ratio affects the results.


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